The intraocular pressure (IOP) of a person's eye has been recognized as an important factor in the early detection of ocular disease. Glaucoma, one such disease occurs quite frequently and can be detected by measuring an increase in intraocular pressure. Using accurate and routine screening, glaucoma may be diagnosed and treated before vision is severely compromised. An increase in intraocular pressure is not sensed by the individual but can be measured in the clinic. Treatment can be started before the onset of gradual, but significant vision loss. Vision loss and blindness can be prevented by early detection. Unfortunately, however, at least one-half the population diagnosed with open-angle glaucoma is asymptomatic. Routine screenings are accordingly instrumental in early diagnosis. The measurement of intraocular pressure is formally known as tonometry—A standard eye test that is done to determine fluid pressure inside the eye.
Tonometry has been practiced for over a half century, but even with this history limitations of IOP monitoring, especially diurnal changes, with presently available tools and methods is recognized:
In normal individuals, IOP varies by 2-6 mmHg over the course of a 24-hour period as aqueous humor production changes. Higher IOP is associated with greater fluctuation and a diurnal fluctuation >10 mmHg is suggestive of glaucoma. Many people reach their peak IOP in the morning hours, but others do so in the afternoon, in the evening, or during sleep; still others follow no reproducible pattern. Comparative evaluation of intraocular pressure with an air-puff tonometer versus a Goldmann applanation tonometer Clin Ophthalmol. 2013; 7: 23-27. Published online 2012 Dec. 27.
Unfortunately, the screening instruments presently available to the medical practitioner are unduly complex. Frequent monitoring, preferably by a patient several times a day, is not available. Often these instruments require frequent recalibration and/or require large and expensive analytical apparatus. They are not suitable for routine measurement that would be required for practical self-use.
In the early history intraocular pressure was determined by two common techniques: indentation or applanation tonometry. In indentation technique, a foot plate with a weight-loaded piston, whose specific weight can be varied, is lowered until it indents the cornea and the foot plate comes to rest on the cornea. The piston is forced upward by the resistance of the cornea being indented. As the piston is forced upward, displacement wias transduced through a lever which actuates a pointer. The pointer then reads out the resistance pressure, or distensibility of the eyeball, providing a value for the intraocular pressure. A representative instrument of this type is the Schiotz tonometer.
The disadvantages of an indentation type instrument are the many sources of error inherent in the instrument itself; i.e., it is affected by temperature, fulcrum friction, accumulation of debris on moving parts causing increased friction. The instrument must be frequently recalibrated. In addition, it is clumsy and difficult to use and inadvertent corneal abrasion may subsequently occur. Indentation produces relatively large volume displacement in the eye, which enhances the effects of varying scleral rigidity. All of these recognized sources of error tend to produce erroneous readings.
Current techniques include Goldmann applanation tonometry, air puff tonometry, ocular response analyzer corneal compensated IOP (ORA IOPcc) and Pentacam corrected IOP. See Shousha et al [Clin Opthamol 2013; 7: 71-79] 2013 for a comparison of these methods. Other methods make use of apparatus such as the Ocuton® S, contact lenses fitted with a strain gauge, the “Home-tonometer” by Zeimer, and Wilensky (analyzing reflected light from a probe-corneal interface), the spring compression Proview® Eye Pressure Monitor (Bausch and Lomb), and Tonopen®. Liang et al (2009 [Sury Ophthalmol 54 (4) July-August 2009]. However, none of these incorporate the desired traits of easy use, low cost and accuracy.
In applanation tonometry, the corneal curvature is flattened or applaned by a flat piston. The piston may be of known weight and the area of applanation determined by an indirect method or it may be of known surface area and the intraocular pressure necessary to applane that area is determined by calculating the force necessary to applane that area. Apparatus using pistons of known surface area are (1) Mackay-Marg which uses electronic means for readout, (2) the Goldmann slit lamp which uses an optical means for readout, and (3) the Tonour which uses a pumped pressure readout.
A Goldmann tonometer uses a small probe attached to a microscope to gently flatten part of one's cornea after numbing drops are used. The apparatus uses a double prism mounted on a standard slit lamp. Using a Goldmann type apparatus, the force required to flatten, or applanate, a constant area of the cornea is measured and related to the IOP using the Imbert-Fick principle. Although or perhaps because of a Goldmann device's complexity, Goldmann is considered by many as the standard for any apparatus to be measured against. However, as advised by Goldmann tonometer marketers: “Applanation tonometry may only be performed by healthcare professionals who are qualified by their respective authorities to do so.” Haag-Streit International.
Another method that is currently popular is dubbed “the air puff method”. Air-puff tonometry is an applanation method where a standardized puff of air flattens a portion of the cornea. The air puff method has an advantage that it is a non-contact method. When properly administered nothing touches the eye thereby avoiding any risk of corneal abrasion. Also, no topical anesthetic is required.
According to Farhood (2013) [Clin Ophthalmol. 2013; 7: 23-27.]:                “The system consists of a central air plenum flanked either side by a light emitter and a light detector. As the pressure of the air pulse directed to the cornea increases to deform the cornea, the corneal surface behaves like a plane mirror, reflecting light to the detector. Corneal applanation is measured by collecting light reflected from the central cornea. A parallel beam of light is directed onto the central cornea at an angle of 30° and the reflected light is measured by a photo detector at an angle of reflection of 30°. The reflected beam of light will be strongest at this angle when the cornea is flat and acting as a plane mirror, rather than as a curved mirror. The instrument records the force of air required to flatten the cornea and displays the IOP that corresponds to that force. The AP tonometer must be used at a set distance from the cornea, and the instrument incorporates an optical alignment system to facilitate this.”        
A suitable for home use method has been tested and used. As reported by Liang et al (2009 [Sury Ophthalmol 54 (4) July-August 2009]:                “The Proview Eye Pressure Monitor (Bausch & Lomb, Rochester, N.Y.) was invented by Fresco in 1997. It is a spring compression apparatus with a 3.06-mm diameter circular tip that is applied to the superonasal orbit over the upper eyelid while the eye is directed inferotemporally. As increasing pressure is applied to the eye, a visual sensation that has been variously described as like a solar eclipse or a dark circle surrounded by a bright halo is produced, and the measured IOP is read off the scale. The visual sensation is an entopic phenomenon that occurs with deformation of the eyeball and had been described by various authors since Alcmaeon of Croton circa 600 BCE.”        
One problem inherent in this method, however, is the overestimation of low intraocular pressures and the underestimation of high intraocular pressures. This underestimation would result in false negative results. Posner advocated use of a tonometer with a piston of pre-determined weight which carries a stain on its contact surface. A print of the stain remaining on the piston after contact is transferred to a paper for measurement. The transfer of area of stain remaining on the foot plate, after corneal contact, to a paper strip, which is then measured on a scale, is very difficult to use and produces many erroneous or unreliable readings.
The Goldmann, Mackay-Marg, air puff, and Tonour methods are complicated, expensive and difficult to use and thus, cannot be routinely used in “group screening” instruments.
It is apparent that these and other similar apparatus cannot provide an inexpensive, easily operated, accurate tool for screening the level of intraocular pressure.
Before tonometry, intraocular pressure (IOP) was clinically evaluated using by palpation through a closed upper eyelid. A skilled ophthalmologist can evaluate approximately whether the IOP is normal, increased or decreased. However, this palpation method suffers from subjectivism, leading to uncertainty of quantifiable results. But the proven efficacy of this qualitative method demonstrates that transpalpebral tonometry can be feasible for screening.
An apparatus currently marketed, the Diaton® tonometer, uses transpalpebral tonometry. The commercial acceptance of this further proves accepted efficacy of transpalpebral methods. However, this apparatus includes an expensive electronic apparatus that is not suited for mass distribution and use.
Patient self-monitoring to date has generally been difficult because most commercial devices cannot be patient actuated—a second person is required. In general the expense of the training and equipment requires the devices to be used in a clinical setting
Phospenes, the creation of a light sensation without interaction of a photon with retina receptors, have been used for in home testing. External pressure on the eye is known to create a visual sensation. However, the correspondence between IOP and phosphenes is not universally accepted. Additionally, studies of this phenomenon have shown poor reproducibility.
In a 2006 writing, Karani et al recognized a need for in-home, patient operated tonometry. [Priya Karani, Shawn Tan, Lin Xiong, & Cen Zhang, Home Intraocular Pressure Measurement Apparatus]
http://bme227.pratt.duke.edu/downloads/S06/IOP_Proposal.pdf] Here they recognized that because of significant diurnal variation IOP measurement, multiple, but infrequent visits to an ophthalmologist throughout the year were not truly indicative of a patient's IOP. Because of this, they recognized a need for a means for a patient could use to self-monitor IOP. Their goal was to provide a tonometry monitor suitable for individual at-home use with a cost of under $100.
Liang et al (2009 [Sury Ophthalmol 54 (4) July-August 2009] also acknowledged the need for a tonometry measuring instrument and associated method for self-measured tonometry:                We can foresee the self-tonometer becoming an important part of the delivery of care to glaucoma patients. Potentially, it will be able to send readings to a secure central database that automatically flags fluctuating IOPs and alerts the treating ophthalmologist electronically. This will provide great benefit to patients who live remote to their treating ophthalmologist and may be useful for population screening programs.        
Partly in response to these recognized needs, the present invention provides an accurate, inexpensive and simple (non-physician, e.g., patient enabled) means for screenings or measurement between clinical office visits. The apparatus is suitably designed so that data can be easily reported electronically to medical professionals who can thereby efficiently monitor a patient's IOP and provide timely modifications to the patient's treatment.